Vinyl monomers such as (meth)acrylate monomers, aromatic vinyl monomers and vinyl ether monomers are widely used as a curable material for an adhesive, a resin syrup, a dental resin, a coating material and a photocurable material. Technologies of mixing commercially available core/shell type fine polymer particles in powder form to the above-mentioned monomers for improving toughness are disclosed in JP-A-4-275204 (Patent Document 1) and Canadian Patent Publication No. 1192688 (Patent Document 2).
It is well known that one particle of such a core/shell polymer in powder form is an aggregate of about several tens m to several hundreds m obtained by aggregating plural core/shell fine polymer particles, for example, plural core/shell fine polymer particles formed to have a primary particle size of about 0.1 m to about 1 m.
When such a core/shell polymer in powder form is mixed to a curable composition comprising a vinyl monomer as a main component by forcedly applying a mechanical shearing force, apparently it can be in a state of being mixed to the composition, but actually the core/shell polymer cannot be dispersed in the form of primary particle and most of the primary particles are present in the vinyl monomer in an aggregated state. In most cases, such a curable composition before cured does not have a viscosity sufficient for transmitting a strong mechanical shearing force to the core/shell polymer being present as an aggregate, and as a result, the core/shell polymer is not in a state of primary particles and is left in the form of an aggregate. In addition, a method for accelerating dispersion of a core/shell polymer by applying heat to a curable composition for avoiding an undesired curing reaction and quality degradation is accompanied by many limitations. Such being the case, graft rubber particles (core/shell polymer) in powder form have been used in a state of being insufficiently dispersed in a curable composition (thermosetting resin), especially in low viscosity vinyl monomers, namely, in a state of being left in the form of an aggregate.
It is well known that in the case of a thermoplastic resin in which a core/shell polymer is easily dispersed in the form of primary particles, the core/shell polymer exhibits a high effect of improving toughness. Therefore, it has been desired that a core/shell polymer is dispersed in the form of primary particles also in the case of improving toughness of a thermosetting resin, namely, a curable composition by using a core/shell polymer. However, while a preferred state of a core/shell polymer before mixed has been proposed in the prior arts, in the light of the above-mentioned technical restrictions, actually it has been extremely difficult to mix a core/shell polymer in a vinyl monomer, to realize a stably dispersed state of the core/shell polymer in the form of primary particles and further to keep this state. Such a problem appears remarkably in the case of using a vinyl monomer selected from the group consisting of a (meth)acrylate monomer obtained from an alcohol having 3 or more carbon atoms and (meth)acrylic acid, an aromatic vinyl monomer and a vinyl ether monomer.
By the way, (meth)acrylic adhesives have been used as adhesives for structural adhesive in a variety of industrial applications since they exhibit an adhesive strength in a short period of time, and for example, have characteristics such as a wide range of allowable mixing ratio of each component, a rapid curing at low temperature and easy handling as compared with epoxy resin adhesives. Thus, (meth)acrylic adhesives are very useful since they can replace conventional riveting and welding by adhesion and can joint materials which have been difficult to joint by riveting and welding, such as jointing of plastic with metal.
For example, Patent Document 3 discloses an acrylic adhesive comprising a (meth)acrylate monomer, a radical polymerization initiator, a mixture of chlorosulfonated polyethylene or sulfonyl chloride and chlorinated polyethylene, and a graft copolymer, and further, there is described that the adhesive preferably comprises a (meth)acrylic acid and an initiator and that a preferred graft copolymer is a core/shell copolymer obtained by grafting MBS or an acrylic rubber with (meth)acrylic acid ester.
Also, Patent Document 4 discloses a two-component adhesive composition having high safety and excellent adhesion and storage stability and comprising:
(1) a (meth)acrylic monomer,
(2) a mixture of chlorosulfonated polyethylene or sulfonyl chloride and chlorinated polyethylene,
(3) an organic peroxide,
(4) a carboxyl group-containing monomer obtained by a reaction of hydroxyalkyl acrylate or hydroxyalkyl methacrylate having an alkylene group having 2 to 4 carbon atoms with polycarboxylic acid or carboxylic anhydride,
(5) a monofunctional epoxy compound,
(6) an antioxidant, and
(7) an amine based activator.
As shown in the above-mentioned Patent Document 3 and Patent Document 4, generally these acrylic adhesives comprise various rubber-like polymers such as chloroprene, styrene/butadiene block copolymer, chlorosulfonated polyethylene and a core/shell polymer having a core of rubber polymer.
Such rubber-like polymers contained in acrylic adhesives are classified into un-crosslinked rubber polymers being soluble in (meth)acrylate monomers, namely, soluble elastomers such as chloroprene, styrene/butadiene block copolymer and chlorosulfonated polyethylene and crosslinked rubber polymers being insoluble in (meth)acrylate monomers such as graft copolymers represented by a core/shell polymer, namely, insoluble elastomers.
A core/shell polymer which is an insoluble elastomer used in Patent Document 3 is one commercially available in powder form, and this core/shell polymer in powder form is added and mixed with an acrylic monomer which is a main component of an acrylic adhesive, to obtain an acrylic adhesive.
It is known that generally such a core/shell polymer is added as an impact resistance improving agent to a thermoplastic resin, and is dispersed in a state of primary particles in a thermoplastic resin by applying a high shearing force during mold-processing, thereby effectively exhibiting an effect of improving toughness of the resin.
By the way, anaerobic cured products such as an anaerobic adhesive and sealing agent are known as one-component curable composition which can be present stably for a long period of time while being kept in contact with air, but immediately starts curing and completes curing in a given period of time once contact with air is cut off for actual use. Examples of such anaerobic adhesives are disclosed in Patent Document 5.
Further, in Patent Document 6, with respect to a liquid gasket to be used for portions requiring sealing property in the field of automobile parts, etc., there is disclosed an anaerobic curable sealing agent comprising urethane(meth)acrylate prepolymer, a radically polymerizable monomer, an organic peroxide, and further core/shell fine particles comprising a core of rubber-like polymer and a shell of glassy polymer as an anaerobic curable sealing agent composition which is excellent in oil resistance and flexibility and does not lose flexibility even for the use under high temperature atmosphere, and it is described that such a sealing agent is excellent in repellency for maintaining flexibility and sealing property sufficient for coping with vibration of a sealing member and external force and have good adhesion on an oily surface.
Namely, the core/shell fine particles in Patent Document 6 are particles of a core/shell polymer and are commercially available as powder form, and the core/shell polymer in powder form is mixed to an anaerobic adhesive to obtain an anaerobic curable composition comprising the core/shell polymer.
It is known that generally such a core/shell polymer in powder form is added as an impact resistance improving agent to a thermoplastic resin, and is dispersed in a state of primary particles in a thermoplastic resin by applying a high shearing force during mold-processing, thereby effectively exhibiting an effect of improving toughness of the resin.
As mentioned above, a core/shell polymer in powder form has been mixed to a (meth)acrylate monomer to be used in a state of paste. However, in this case, core/shell particles could not be dispersed in the form of primary particles and were used as they were. The mixture can be apparently in a mixed state, but actually a core/shell polymer cannot be dispersed in the form of primary particles and most of primary particles are present in a vinyl monomer in an aggregated state. It is well known that in a thermoplastic resin in which a core/shell polymer is easily dispersed in the form of primary particles, a core/shell polymer exhibit a high effect of improving toughness, and it is important to disperse a core/shell polymer in the form of primary particles. However, in prior arts, a proposal on a preferred state of a core/shell polymer before mixing has been made, but it has been extremely difficult to actually mix a core/shell polymer to a vinyl monomer to realize a state of being stably dispersed in the form of primary particles and further to maintain this state.
In the light of such a technical background as mentioned above, in an anaerobic acrylic adhesive composition comprising a core/shell polymer, since the core/shell polymer was in an aggregated state and was not dispersed in the form of primary particles, inherent performance thereof could not be exhibited and in this point, there was room for improvement.
By the way, photocurable adhesives have been used for assembly of various industrial materials because they function as an adhesive to be rapidly cured by irradiation of ultraviolet ray, etc. and also have been widely used industrially as excellent adhesives since articles to be adhered can be fixed in a short period of time and an energy amount necessary for curing is small.
Patent document 7 discloses a technique of using a photocurable resin composition comprising core/shell fine resin particles comprising a core particle having elasticity of rubber and a shell layer formed on the core particle surface in the photocurable resin, as a sealing agent and a sealing material having improved adhesion with a glass substrate.